The present invention relates to a printing process for forming a printed image directly on a printing medium and an apparatus therefor, more specifically, the present invention relates to an ink jet printing process and a printing apparatus, where a cleaning mechanism is provided to the ink feed line in an ink jet system of ejecting an oil using an electrostatic field and thereby, a high-quality printed image and a high-speed printing can be obtained.
The printing process for forming a printed image on a printing medium based on image data signals includes an electrophotographic method, a sublimation-type or melting-type heat-transfer method and an ink jet method.
The electrophotographic method requires a process of forming an electrostatic latent image on a photoreceptor drum by electrification and exposure and therefore, suffers from complicated system and expensive apparatus.
The heat-transfer method uses an ink ribbon and therefore, despite its inexpensive apparatus, suffers from high running cost and treatment of a waste material.
The ink jet method performs the printing directly on a printing medium by ejecting an ink only on a desired image area using an inexpensive apparatus and therefore, ensures efficient use of the coloring agent and low running cost.
With respect to the method for applying the ink jet technology to printing system, for example, JP-A-10-286939 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a process for additionally printing variable numbers, marks or the like on the same printing paper using the ink jet system by providing an ink jet printing apparatus to a rotary printing press.
The printing of image information is preferably in a level as high as comparable to the photographic image, however, conventional ink technologies of pressure-ejecting an aqueous or organic solvent-type ink containing a dye or pigment as a coloring agent is disadvantageous in that since a droplet containing a large amount of a solvent is ejected, unless expensive exclusive paper is used, the printed image blurs.
Accordingly, in the case of performing the printing on a normal printing paper, a plastic sheet as a non-absorptive medium, or the like, a high-quality printed image cannot be obtained.
As one of the ink jet technologies, a method of heat-melting an ink which is solid at an ordinary temperature, and jetting out the obtained liquid ink to form an image is known. When this ink is used, blurring of the printed image may be reduced, however, because of high viscosity of the ink at the ejection, a fine droplet cannot be jetted out and the obtained individual dot images are large in both the area and the thickness, as a result, a high-precision image cannot be formed.
A printing process and a printing apparatus using an ink jet method of ejecting an oil ink using electrostatic field are attracting an attention. In this ink jet method, the ejection performance is deteriorated by the attachment of dusts and fixing of ink to the ejection electrode. It is also found that the fixing of ink in the passage of ink circulation system causes reduction in the ink flow rate and this deteriorates the ejection performance.
The problem to be solved by the present invention is to provide a method for removing attachments adhering to the ejection electrode and passage of the ink circulation line.
The present invention has been made by taking account of the above-described problems.
Accordingly, an object of the present invention is to provide an ink jet printing process which can print a printed matter having a clear and high-quality image by an inexpensive apparatus and a simple and easy method and moreover, which can remove the attachments adhering to the electrode and passages of the circulation line.
Other objects and effects of the invention will become apparent from the following description.
The above-described objects of the present invention have been achieved by providing the following processes and apparatuses.
1) An ink jet printing process comprising:
forming an image directly on a printing medium by an electrostatic ink jet method comprising ejecting an oil ink using electrostatic field based on signals of image data; and
preparing a printed matter by fixing said image,
wherein said process uses:
(1) an ink circulation line having the following members a, b and c:
a, an ink jet ejection head,
b. an ink transportation line comprising an ink feed line for feeding said oil ink to said ink jet ejection head and an ink recovery line for recovering said oil ink from said ink jet ejection head, and
c. an ink tank for storing said oil ink;
(2) a cleaning solution feed line for feeding a cleaning solution to said ink transportation line; and
(3) a cleaning solution recovery line for recovering said cleaning solution from said ink transportation line, and
wherein at the time of cleaning, said ink tank is separated from said ink transportation line, said cleaning solution feed line and said cleaning solution recovery line are connected to said ink transportation line, and a cleaning solution is transported to the ink transportation line to perform the cleaning.
2) The ink jet printing process according to item 1) above, wherein said cleaning solution is circulated at the time of cleaning.
3) The ink jet printing process according to item 1) or 2) above, wherein the flow rate of said cleaning solution at the time of cleaning is higher than the flow rate of ink at the time of drawing an image.
4) The ink jet printing process according to any one of items 1) to 3) above, wherein said oil ink comprises:
a nonaqueous solvent having an electric resistivity of 109 xcexa9cm or more and a dielectric constant of 3.5 or less; and
resin particles dispersed in said nonaqueous solvent, said resin particles being colored.
5) A printing apparatus comprising:
image-forming unit which forms an image directly on a printing medium based on signals of image data; and
image-fixing unit which fixes the image formed by said image-forming unit to obtain a printed matter,
wherein said image-forming unit comprises:
an ink jet drawing device having an ink jet ejection head from which an oil ink is ejected using electrostatic field,
an ink circulation unit comprising:
(1) said ink jet ejection head,
(2) an ink transportation unit comprising an ink feed member which feeds said oil ink to said ink jet ejection head and an ink recovery member which recovers said oil ink from said ink jet ejection head, and
(3) an ink tank for storing said oil ink,
a cleaning solution feed member which feeds said cleaning solution to said ink transportation unit,
a cleaning solution recovery member which recovers said cleaning solution from said ink transportation unit,
a feed side changeover member which separates said ink tank from said ink transportation unit and at the same time, connecting said cleaning solution feed member to said ink transportation unit, and
a recovery side changeover member which connects said cleaning solution recovery member to said ink transportation unit.
6) The printing apparatus according to item 5) above, further comprising a circulation unit which circulates said cleaning solution at the time of cleaning.
7) The printing apparatus according to item 5) or 6) above, further comprising a flow rate-varying member capable of increasing the flow rate of said cleaning solution at the time of cleaning higher than the flow rate of ink at the time of drawing an image.
8) The printing apparatus according to any one of items 5) to 7) above, wherein said oil ink comprises:
a nonaqueous solvent having an electric resistivity of 109 xcexa9cm or more and a dielectric constant of 3.5 or less; and
resin particles dispersed in said nonaqueous solvent, said resin particles being colored.
9) The printing apparatus according to any one of items 5) to 8) above, further comprising a dust-removing member which removes dusts present on the surface of said printing medium at least one of before and during the printing on said printing medium.
10) The printing apparatus according to any one of items 5) to 9) above, further comprising an opposing drum which is disposed at the position facing said ejection head and which is rotatable and capable of mounting a printing medium thereon, so that the drawing of an image can be performed while moving said printing medium by the rotation of said opposing drum.
11) The printing apparatus according to item 10) above, wherein said ejection head comprises a single channel head or a multi-channel head and is movable in a direction parallel to an axis of said opposing drum.
12) The printing apparatus according to any one of items 5) to 9) above, further comprising at least a pair of capstan rollers capable of holding and running said printing medium.
13) The printing apparatus according to item 12) above, wherein said ejection head comprises a single channel head or a multi-channel head and is movable in a direction orthogonal to the running direction of said printing medium.
14) The printing apparatus according to item 10) or 12) above, wherein said ejection head comprises a full line head having almost the same length as the width of said printing medium.
15) The printing apparatus according to any one of items 5) to 14) above, wherein said ink jet drawing device has a stirring member which stirs said oil ink in the ink tank for storing said oil ink.
16) The printing apparatus according to any one of items 5) to 15) above, wherein said ink jet drawing device has an ink temperature-controlling member which controls the temperature of said oil ink in the ink tank for storing said oil ink.
17) The printing apparatus according to any one of items 5) to 16) above, wherein said ink jet drawing device has a concentration-controlling member which controls the concentration of said oil ink.
18) The printing apparatus according to any one of items 5) to 17) above, which comprises a cleaning member which cleans said ejection head.
As such, the printing apparatus of the present invention is characterized in that in the ink jet printing apparatus of ejecting an oil ink using electrostatic field, the oil ink in the ink passage of ink circulation line is changed over to the cleaning solution at the time of cleaning. By this changeover, attachments adhering to these electrode and passage of the circulation line can be removed.